1. Field of the Invention
The present invention relates to an optical disc tracked by a sampled servo system and a driving apparatus for the optical disc, and more particularly to an improvement in a format of pits for sampled servo preformatted on the optical disc.
2. Description of the Prior Art
An optical disc has widely been used as a large capacity storage recordable in high density. The recording method by the optical disc is to record pits spirally or concentrically on a recording film of the optical disc by the laser beam restricted to about 1 .mu.m in diameter. It is required for scanning the optical disc by the light beam in high density and recording pits to apply tracking servo and focus servo for coinciding the focus of light beam with the target position on a recording film. There are two methods for carrying out the tracking servo and focus servo. A first method is a continuous servo system using a guide groove and a second method a sampled servo system using wobbled pits.
FIG. 1 shows a track sector format of the optical disc of a conventional sampled servo system disclosed in, for example, SPIE, Vol. 695, Optical Mass Data Storage 2 (1986), Page 112. In FIG. 1, reference numeral 90 designates a sector construction of one circle of a track, each track comprising 32 sectors (labelled No. 0 to No. 31). Reference numeral 91 designates a segment construction of one sector, the one sector comprising 43 segments (B1 to B43). Each segment is composed of 18 bytes in total, including of a servo area of 2 bytes and a data area 16 bytes in continuation thereof. Each track is divided into 32.times.43=1376 segments.
FIG. 2 shows a pit pattern of the servo area. The pits 92, 94 and 93, 94 are slightly shifted reversely to the track centers 97 and 98, so that a tracking sensor signal is obtainable only from a pair of the pits (the pair is called wobbled pits). The principle of operation of such sampled servo system is described in, for example, SPIE, Vol. 529, Third International Conference on Optical Mass Data Storage (1985), Page 140. In such a conventional sampled servo system, since the tracking sensor signal can be obtained from pit pairs in the servo area, a guide groove for tracking is not required. Therefore, in order to have access at high speed to a target track from the present track, as shown in FIG. 2, servo area constructions A and B are alternately disposed at every 16 tracks so as to enable a moved track quantity to be counted.
In FIG. 2, track number is given in EQU Track Number=I+(N-1).times.16,
where I=1, 2, 3, . . . , 16. In the servo area construction A, N=1, 3, 5 . . . and in that B, N=2, 4, 6 . . . . At the servo area constructions A and B, one pair of pits 92 and 93 are shifted in position in the direction of the track. During the access operation while the track is being traversed obliquely, the position of the pit is detected, thereby enabling the traversed track quantity to be obtained. This state will be explained in FIG. 3.
In FIG. 3, reference numeral 67 designates a large number of centers of the tracks, which are spaced from each other at intervals of 1.5 .mu.m. Reference numeral 68 designates position of servo area. The servo area constructions A and B, as shown at the right-hand end in FIG. 3, are represented by A and B at every 16 tracks. Reference numeral 69 designates the locus of the light spot when it has access at high speed, a black point 70 representing the point where the light spot intersects the servo area, and the servo area construction being recognized at the point 70. The recognized signal waveform is shown by reference numeral 99. The high level shows the servo area, construction A and the low level shows servo area construction B. This means that sixteen tracks are counted at every change of state of the signal waveform 99. Thus the number of traverse tracks are countable from the signal waveforms 99 in access operation, resulting in that the light spot reaches the target track.
When reaching the target track, the tracking control using the reproducing signal for the wobbled pit pairs is carried out.
FIG. 4 shows a waveform of reproducing signals near the servo area, in which the reproducing signals 46 and 47 of the wobbled pits 92 and 94, the reproducing signal 48 of the clock pit 95 and the reproducing signal 49 of data are shown. FIG. 5 shows the principle for obtaining a tracking error signal for the tracking servo using the reproducing signals 46, 47 and 48. FIG. 5-(a) shows a reproducing signal when the light beam shifts leftwardly with respect to the track and performs the tracking, in which the reproducing signal 46 of the wobbled pit is reproduced large and the reproducing signal 47 of the wobbled pit is reproduced small. FIG. 5-(c) shows the waveform when the light beam is traced rightwardly of the track, the reproducing signal 47 being largely reproduced. FIG. 5-(b) shows the waveform when the light beam carries out tracking on the axis of track, in which the reproducing signals 46 and 47 are equally reproduced, the reproducing signal 48 of the clock pit being reproduced at a maximum. Accordingly, when a difference between the reproducing signals of both of the wobbled pits is taken, the tracking error signal is obtainable. FIG. 6 is a circuit diagram of tracking servo in the sampled servo system for performing the tracking servo by use of such signal, in which the signal shown in FIg. 4, read out from an optical head (not shown) and amplified by a preamplifier (not shown), is inputted to an input terminal 50. The input signal is switched by switches 51 and 52 and a maximum value of the reproducing signals 46 and 47 of wobbled pit is picked-up, and is held in capacitors 54 and 55. Switches 51 and 52 are driven by a clock circuit 53, so that the switch 51 is closed only at the point of time when the maximum value of reproducing signal 46 of the wobbled pit is developed and holds the maximum value in the capacitor 54, the switch 52 being closed similarly when the maximum value of reproducing signal 47 of the wobbled pit is developed. The clock circusit 53 is adapted to respond to the clock pulse 48 of input signal and operate in synchronism with rotation of the disc, thereby enabling the above-mentioned operation. A differential amplifier 56 produces a difference signal from voltages held by the capacitors 54 and 55, the difference signal being a tracking error signal. The tracking error signal is amplified by an amplifier 57 to drive an actuator 58 for the optical head. The actuator 58 carries thereon an objective lens (not shown), so that the tracking error signal drives the actuator 58 to move the objective lens so as to allow the light beam to trace the center line of track, thereby constituting the tracking servo system.
Next, explanation will be given on the data reproducing operation.
FIG. 7 is a block diagram of the conventional optical disc apparatus for forming the reproducing clock and data when the signal format in FIG. 2 is used. FIG. 8 illustrates the waveforms of the above. A photosensor current is I/V-converted and its reproducing voltage waveform (a) is inputted to a terminal 20 and then its pit tip is detected by a detector 21 to thereby obtain a detection signal (b). A gate circuit 23 utilizing characteristics between the clock pit and the wobbled pit and of that between the continued clock pits and detecting only the clock pit signal, is used to obtain a detected signal (c) from the detected signal (b). A phase-locked loop (PLL) circuit 24 produces a recording-reproducing clock (to be hereinafter called the rec-rep CK) (d) from the detected clock pit. Also, a switching signal (e) for a servo area and data area is obtained. The rec-rep CK is used to digitally modulate by a modulator 27 the recording data input to a terminal 26. The modulated clock and modulated data are reversely issued to terminals 30 and 31, and a laser recording amplifier records them when only the switching signal of data area is obtained. The recording method includes an RZ recording system (recording waveform (g)) for recording in a fixed length pulse of bit of logical "1" at the modulation data (f) and an NRZI recording system (recording waveform (i) for inverting its polarity at bit of logical "1". Each reproducing waveform becomes Gaussian isolated reproducing waveform (h) and its level tip is differential-detected, thereby obtaining a detected signal (k). On the other hand, in the NRZI system, reproducing waveform (j) is two-value-level-sliced so that its edge information is the detected signal (k). The detected signal (k) is data-decoded by the rec-rep CK (d) and a decoder 25 outputs decoded clock (d) and decoded data 1 to terminals 28 and 29. As above-mentioned, at the sampled servo system optical disc, since the recording data are written and the reproducing data are detected by using the clock pit being preformatted of the rec-rep CK, the recording code does not require the self clock capacity, such as the conventional run-length-limited code, and also it is advantageous in that a clock shift problematical in the self clock is difficult to occur. Conversely, there is no self clock capacity because of the data area being divided by the servo byte. When the phases of reproduce detected signal and that of the rec-rep CK shift, there is no decoding allowance to cause a detection error. This phenomenon will be described in FIG. 9. In FIG. 9, (a) shows the rec-rep CK; (b), an RZ recorded signal; and (c), an NRZI recorded signal. The detected signal, when detected at the center of a decoding window Tw of the rec-rep CK, has a width of the detecting decoding window of .+-.Tw/2 and is most difficult to cause the detection error. Next, the detected signal of the optical disc will be considered. Conventionally, a punching write-once type optical disc has used the RZ recording system. When the recording power is normal, the recorded signal phase and reproduced signal phase are relatively difficult to cause a shift, so that the reproduce detected signal can be reproduced in normal phase, thereby enabling the reproduce detected signal can be reproduced in the substantially normal phase as shown in FIG. 9-(b). However, the laser power of optical disc changes in its current/power characteristic with respect to the temperature and a shift of power characteristic or a change in the power characteristic at the inner and outer peripheries of the disc cause a phase shift, which remarkably affects a rewritable type magneto-optical disc. During the magneto-optical recording, when the laser power is applied on the disc and the temperature exceeds the predetermined value (curie temperature) with respect to the polarity of magnetic field, a magnetic film is magnetized to the applied magnetic field polarity, so that the rotation of disc after the stop of power application allows the magnetized polarity to remain when the disc is cooled under the curie temperature. Therefore, the residual magnetization pattern delays with respect to the applying power so that, when reproduced, the phase of reproduce detected signal leads with respect to the rec-rep CK.
In order to reduce the latency by the erase and recording mode recently required for the conventional magneto-optical disc, the system of magneto-optical disc having an immediate overwrite function has been proposed. The highest realizable system is a magnetic field modulation overwrite system, which uses the laser power as the DC recording power during the recording and changes the magnetic field polarity by a recording modulation signal, thereby performing the erasing and recording in the same mode. This system eliminates the defect in the optical disc to enable the overwrite with respect to the magnetic disc. At this time, the NRZI recording system carries out the magnetic field modulation so as to be realizable of largely improving the recording density. The reason for this is that when the minimum pit length can be recorded at the same width, the recording density at the NRZI recording is improved two times larger with respect to the RZ recording. However, it is reported that when the magnetic field modulation system is adopted to the sampled servo type optical disc, phase with respect to recording-reproducing of the reproduce detected signal largely changes.